Method and means for reducing hydrodynamic noise



Dec. 15, 1964 M. H. SILVERMAN METHOD AND MEANS FOR REDUCING HYDRODYNAMICNOISE Filed Feb. 14, 1963 FREQUENCY IN KC/SEC Fig.3

HYDROFOI L TRAILING EDGE Fig.4

SURFACE PORTION O YDROF B T ETRE D FREQUENCY IN KC/SEC E o 1].] O. (n gm 2 a: v E i *Al -AFTER FIN TREATMENT 6 INVENTOR MURRAY H. SILVERMAN C75 A TTORm United States Patent Ofifice 3,161,167 METHOD AND MEANS FORREDUCING HYDRODYNAMIC NOISE Murray H. Silver-man, Washington, D.C.,assignor, by mesne assignments, to the United States of America asrepresented by the Secretary of the Navy Filed Feb. 14, 1963, Ser. No.258,971 4 Claims. '(Cl. 114-20) The present invention relates in generalto a method and means for eliminating, or materially reducing,hydrodynamic noise resulting from the passage of an object at relativelyhigh speed through a fluid meduim. In a preferred embodiment, theinvention relates to lowering the noise level produced by movement of anunderwater torpedo so that the latter is rendered more difficult ofdetection by an enemy vessel toward which the torpedo has been launched.

The presence of hydrodynamic noise constitutes one of the major problemsinvolved in the operation of modern high-speed underwater missiles. Inmany instances this noise is of sufiicient amplitude so that the missileis detectable by the enemy in time to allow evasive maneuvers to besuccessfully carried out. Consequently, it is desirable to eliminate tothe maximum possible extent the external noise developed by such amissile in order that its combat efficiency may be increased.

One of the moreserious forms of hydrodynamic noise is singing noise andit is with this particular phenomenon that the present invention isconcerned. Singing is of particular importance since it is tonal innature and is usually very intense in water. Such a tone cansignificantly raise the detection level of an otherwise relative lownoise energy spectrum. Singing is caused by the same physical phenomenonas the commonly noted vibration of iced telephone wires or car aerialsin high winds when their axes are transverse to the wind. Although theflow ahead of the wire is relatively smooth, there is an unsteady fiowin its. wake caused by vortices periodically shed alternately above andbelow the wire. This unsteady flow is known as the Von Karman' vortexstreet and produces an oscillatory force resulting in singing of thewire. In other bodies, if the frequency of one of its natural modescoincides with the frequency of the street, singing will occur.

It has been found that this undesirable effect is due in large measureto the design of certain auxiliary fins located on the aft section ofthe torpedo just forward of the propellers. employed primarily to createa lift in the proper direction, which tends to reduce the attack angleresulting from the static turning moment inherent in the torpedo duringits passage through the water. However, because of the initially severestability requirements of a torpedo of the type being described,additional fins are desirable, and the latter are positioned at a 45angle to the cruciform fins. Although-these added fins are designed forstability and hence possess a rather fiat profile, nevertheless theywere found to. sing in such fashion that asubstantially constant puretone was produced of a nature readily detectable by an enemy.

In a copending United States patent application of the presentapplicant, Serial No. 174,323 filed on February 19, 1962, and entitledHydrodynamic Noise Reduction, there is disclosed a method and means foreliminating,

The usual cruciform fins on a torepdo are' 3,161,167 Patented Dec. 15, 1964 or at least materially reducing, the over all noise output of thetorpedo when it passes at relatively high speed through a fluid medium.The solution therein disclosed consists in modifying the profile of each45 fin by removing a terminal portion of the fin material so as to forma wedge having a pretermined angle, and then extending the Wedge slopeforwardly to intersect the respective fin surfaces at a pre-se-lectedpoint. This results in the angle actually seen by the fluid stream linesbeing relatively small because of the symmetric configuration of the finprofile.

While the above expedient is in every way satisfactory insofar as noisereduction is concerned, it has now bee-n discovered that it is notnecessary to actually alter the fin profile in order to achieve thedesired results. Alternatively, it is feasible to treat the surface ofthe fin in the vicinity of its trailing edge so that, instead of beingsmooth, this particular fin surface area is rough. Con- Other objectsand many of the attendant advantages of i this invention will be readilyappreciated as the same becomes better understood by reference to thefollowing detailed description when consider-ed in connection with theaccompanying drawings wherein:

FIG. 1 is a side view of an underwater torpedo of the type to which thepresent invention is particularly applicable;

FIG. 2 is an aft view of the missile of FIG. 1, showing the angularrelationship of the various fins with which the missile is provided;

FIGS. 3 and 6 are graphs illustrating the change in noise output of themissile of FIG. 1 when the 45 fins thereof have a surface portiontreated in accordance with the invention;

FIG. 4 is a view showing the profile of 'one of the 45 fins illustratedin FIGS. 1 and 2; and

FIG. 5 is an enlarged perspective view of a portion of the fin of FIG.showing one manner in which a surface area of the fin may be treated inaccordance with the principles of the present invention. I

Illustrated in FIGS. 1 and 2 of the drawings is an underwater missile ortorpedo which is designed to be guided toward a target by controlinformation supplied to 'the torpedo over a wire 8 which trails behindthe .missile in the manner shown. The torpedo of FIGS. 1 and-2,thereforeyis formed with a body 10 which cona source of energy forcarrying out its prescr'ibed'functions.

However, these'components form no part of the present invention,and'hence neither an illustration nor a detailed description-thereofisdeemed necessary. It need only be noted that the torpedo of FIGS. land2 is driven by a pair of propellers 12, and, for purposes 3 ofstability, is provided with a plurality of fins 14 which are spacedapart circumferentially about the longitudinal axis of the missile in amanner best brought out by HQ. 2. In addition to the usual cruciformfins 16, a plurality of auxiliary fins 18 are employed, each of thesefins 18 being disposed at an angle of 45 to the principal cruciform fins16 as clearly brought out by FIG. 2. It has been determined duringactual operation of the torpedo being described that one of theprincipal sources of hydrodynamic noise is the singing of the fins 14 asthe result of oscillatory forces produced by the unsteady fluid fiowpast the fins 18, particularly when the missile is traveling at arelatively high speed. There is an interdependency among the effectivedepth of the fin trailing edge, fiuid velocity and oscillationfrequency, and this relationship is designated by the Strouhal Number.This Strou'hal Number is defined as where f is the oscillationfrequency, d is the effective depth of trailing edge of the fin (forcylinders and spheres a is the diameter) and v is the velocity of thefluid. As above mentioned, singing of the fins 18 is in the nature of asteady or constant tone, and, since this tone is very intense in wateror other fluid, it is readily detectable by a device such as ahydrophone.

The interdependency of the factors involved permits the frequency of theexciting force to be changed by varying the thickness of the trailingedge of the fin and/ or its boundary layer, and, in the same manner, achange in the fins structural characteristics can also change itsresonant frequency. In such situations, the quantity d is usuallyconsidered to be a function (the thickness) of the trailing edge of thefin.

Applying the above approach to the particular torpedo design set forthin FIGS. 1 and 2 of the drawings, the initial factor to be determined isthe frequency at which the singing of the fins occurred. Thisinformation is readily derived, and, as shown in FIG. 3 of the drawings,a high noise peak is present in one particular region of the frequencyspectrum. Utilizing the formula above given showed that this finexcitation occurred in the last .30 inch of each 45 fin and that, ifexcitation could be eliminated in this region, practically all of theobjectionable singing would disappear. Following this avenue ofapproach, the terminal .30 inch on both surfaces of each of the 45 finswas modified as shown in FIGS. 4 and 5 by applying to each such surfacea thin coating of an adhesive in which were dispersed minute particlesof silica. It was found that these silica or sand particles disturbedthe fluid flow over the trailing edge of each fin sufficiently togreatly reduce the noise level. Since a similar modification to each ofthe 90 cruciform fins produced no additional improvement, it wasconcluded that only the 45 fins were contributing to the hydrodynamicnoise eifect. This discrepancy in the mode of vibration of the 45 and 90fins might possibly be due in part to the fact that whereas all of the45 fins Were 8.5 inches long in the example being given, the uppervertical fin, on the other hand, was 13.5 inches long and the lowervertical fin 48.5 inches long.

While the particular sand employed was ordinary silica sand, any sand ofanalogous size is'satisfactory. The

fin area to be treated is first covered with a thin coating of glue suchas a rubber-based cement. The sand is then preferably sprinkled on whilethe glue is wet. The sand should be applied so that the particles areneither too densely distributed (each sand particle in direct contactwith adjacent particles) nor too sparsely distributed (to result in widestreaks or open patches) but intermediate these extremes. Only one layerof sand is necessary. It has been found in practice that toothick, roughor dense a layer will cause cavitation and defeat the purpose of theinvention. a

Extensive tests of a torpedo, the 45 fins of which were treated inaccordance with this procedure, showed essentially no extreme peaks ofoscillation, and the noise output throughout the frequency range wasessentially uniform as brought out in FIG. 6 of the drawings.Consequently, detection of such a projectile by an enemy is renderedmuch more difficult than was the case prior to applicantsinvestigations.

Since the purpose of sanding a portion of the fin surface area is toroughen the trailing edge of a hydrofoil, the same objective may beachieved by:

(l) Employing any solid particles of substantially homogeneousdistribution and of a diameter comparable to silica sand, and utilizingany suitable adhesive which will preclude particle slippage ordetachment during actual use, or

(2) Roughening the desired surface area of the fin by any knownmetal-forming or shaping process which is capable of yielding surfacecharacteristics similar to those of a sanded fin, such, for example, asby embossing or removing small areas of fin material to create astippled effect.

It will be immediately recognized that the chordal distance over theplanar areas of which the disclosed treatment is to be applied (thedistance A-B in FIG. 5) will vary widely depending upon the dimensionsand configuration of the hydrofoil and upon the operational speed of theprojectile. In all cases, however, it is the trailing edge of themissile fin which is to be modified in the manner set forth.

The process herein described is readily carried out by a minimum ofequipment and at a relatively low cost in terms "of both materials andmanpower. A substantially complete elimination of the Von Karman vortexstreet in the manner taught does not measurably increase the cavitationeffect which, when present to any appreciable degree, is itself a sourceof hydrodynamic noise.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

I claim:

1. The method of reducing the hydrodynamic noise level of an underwatertorpedo having at least one hydrofoil, said method including the step ofroughening the surface area of such hydrofoil adjacent the trailing edgethereof so as to disturb the fluid flow over such surface when saidtorpedo is in motion, the said surface area of the hydrofoil beingroughened by applying a granular substance thereto as an outer coating.

2. The method of reducing the hydrodynamic noise level of a missiledesigned to pass through a liquid medium during operation, said missilebeing stablized by a plurality of longitudinally-disposed fins arrangedcircumferentialy in spaced-apart relation, certain of which fins form anangle of 45 to the vertical when said missile is in operation and which45 fins each create an oscillatory turbulence in the wake thereof duringpassage of said torpedo through said liquid medium, such turbulencebringing about a vibration of the 45 fins when the natural resonantfrequency of the latter approaches the turbulence frequency, said methodincluding the step of toughening a portion of the surface of each 45 finadjacent the trailing edge thereof to modify the fluid flow over suchroughened fin surface and hence alter the frequency of the resultingturbulence.

3. The method of claim 2 in which the chordal distance over theplanar'areas of which each 45 fin is to be roughened is determined inaccordance with the following formula 5 6 Where References Cited by theExaminer f=the turbulence frequency d =the elfective thickness of the 45fin aft portion UNITED STATES PATENTS v=the velocity of the fluid 1,7 ,49 Crook 244-75 The method of fabn'cafing'a P f qesigned g 5 513331321653; iiiiiit 'jjjjjjj:III-2533353 use on an underwater torpedo Whlchcornpnses coating 7 3,076,725 2/63 Boggs an aft portlon of the hydrofo1lsurface u ith a fluid ad- 3,096,739 7/63 Smith hesive, applying .to saidadhesive a comnnnuted material a in such a manner that the particlesthereof are substan- FOREIGN PATENTS tialy evenly distributed over thesaid hydrofoil surface 10 7 490,501 8/38 Great Britain. portion, andthen alowing the fluid adhesive to harden 580,806 9/46 Great Britain.

soas to'retain the particles of comminuted material in the respectivelocations Where they were originally BENJAMLN BORCHELT Pnmary Exammel"applied. 15 SAMUEL. FEINBERG, Examiner.

1. THE METHOD OF REDUCING THE HYDRODYNAMIC NOISE LEVEL OF AN UNDERWATERTORPEDO HAVING AT LEAST ONE HYDROFOIL, SAID METHOD INCLUDING THE STEP OFROUGHENING THE SURFACE AREA OF SUCH HYDROFOIL ADJACENT THE TRAILING EDGETHEREOF SO AS TO DISTURB THE FLUID FLOW OVER SUCH SURFACE WHEN SAIDTORPEDO IS IN MOTION, THE SAID SURFACE AREA OF THE HYDROFOIL BEINGROUGHENED BY APPLYING A GRANULAR SUBSTANCE THERETO AS AN OUTER COATING.